Caf rejected the protest on a technicality at the weekend, saying, among other things, that it was not written on the correct form.

The Namibia Football Association says it is prepared to take their case to the Court of Arbitration for Sport if necessary.

"It is the responsibility of the association intending to summon and field a player for one of its representative teams to examine if the player is eligible to play for them or not," Fifa told Reuters in a statement.

"This means that they have to verify if all the rules are followed.

"We cannot comment on that (Caf) decision."

"Neither are we in a position to establish whether the player is or is not eligible to play for the representative teams of Burkina Faso, since, as already mentioned previously, this is the responsibility of the association."

Burkina Faso had at one point approached Fifa over the player's eligibility but said it "did not receive any follow-up from the association concerned".

The Stallions, who have qualified for next year's Nations Cup, said Zengue, who plays for Russian Premier League side Terek Grozny, had been given a passport because he was married to a Burkinabe women.

However, Fifa statutes say that, in addition to being granted nationality, a naturalised player must have lived in his adopted country for at least five years.

Fifa altered the regulations in 2004 to avoid situations where players simply needed to obtain a passport to play for their adopted nation.